The present invention relates to devices and methods for removing distortion from voice processing systems, and more particularly to a sign bit integrator and method that correct offsets in telephone system components that distort a voice signal.
Systems that process voice signals include a number of components that receive direct current inputs that can cause voice signal distortion. Voice signal processing systems assume that the voice signal is composed of sine waves that are symmetrical with the same number of negative and positive excursions. However, some components, such as amplifiers and comparators, receive direct current inputs that can cause the components to drift so that the signal is no longer symmetrical, causing distortion of the voice signal. The direct current inputs, denoted offsets, may be counterbalanced by inserting a corrective signal. The present invention is directed to generation of such corrective signals.
While offset correction is desirable in a variety of voice processing systems, it has been found that correction is especially desirable in telephone systems where distortion caused by offsets may seriously degrade signal quality. Offsets are unavoidably inserted in various telephone system components, and it has been found that correction of these offsets is particularly effective when performed in coder/decoders (CODECs) located at the interface between the subscriber and the central office.
In a CODEC, a sign bit integrator may be used to insert the offset corrections. The sign bit integrator continuously samples the sign bits of received digital voice signals and averages them over time to determine whether offsets are affecting the symmetry of the positive and negative excursions of the voice signal. The lowest frequency that is handled by a CODEC is about 200 Hz, and sign bit sampling typically takes place at a frequency of about 8 kHz. Thus, there may be up to 40 samples per cycle and about 20 samples before the polarity of an input signal reverses. In a perfect (no offset) system, the number of samples of each polarity would be same. However, in practice such perfect systems do not exist and offset corrections are required.
Accordingly, it is an object of the present invention to provide a novel sign bit integrator and method for correcting offsets in a voice signal processing system.
It is another object of the present invention to provide a novel sign bit integrator and method for generating an offset correction signal having a polarity opposite the polarity of the offset.
It is yet another object of the present invention to provide a novel sign bit integrator and method for generating an offset correction signal with a variable magnitude so that the speed with which the offset is corrected may be selected.
It is still another object of the present invention to provide a novel sign bit integrator and method for generating an offset correction signal in which a current mirror for setting the magnitude of the corrective signal includes a pair of transistors with a ratio of active area sizes selected to set the magnitude.
It is a further object of the present invention to provide a novel sign bit integrator for generating an offset correction signal in which a current mirror for setting the magnitude of the offset correction signal includes plural transistors with diverse ratios of active area sizes and a switch for selecting a pair of the transistors to set the magnitude.
It is yet a further object of the present invention to provide a novel sign bit integrator and method for generating a signal for correcting distortion in a voice signal in a telephone system CODEC in which an inverter generates an output when a voice signal is sampled in the CODEC, a load capacitor generates a charge pulse upon receipt of the output from the inverter, a current mirror sets a magnitude and polarity of the charge pulse, and an integrator capacitor receives the charge pulse and provides it as an offset correction signal to the CODEC.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.